Neutron Reflection and Counting

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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TLGNick
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Neutron Reflection and Counting

Post by TLGNick » Tue Sep 27, 2005 5:05 am

The only good thing about a long jet ride on a business trip is the time is affords one to catch up on reading. I flew from Boston to Dallas and Dallas to Guad, Mexico and back last week. During the trip down and back I managed to finish... just as the plane was landing back at Logan in Boston, "The Curve of Binding Energy" by John McPhee.

The book although dated was a good reading and interesting story of Ted Taylor a theoretical physicist who was one of the most inventive nuclear scientist of our time. He miniaturized the atomic bomb, and designed the largest yield fission bomb every exploded. He also contributed to the Fusion bomb and led a scientific effort to build a nuclear-powered spaceship. He also led the fight for safer controls due to the alarmingly available weapons grade uranium and plutonium. He was convinced at a time when others were not that an amateur could make a nuclear bomb.

There were a number of interesting things in the book but one that struck me was that Taylor talked about the importance of "neutron reflection" in the bomb design. He stated that steel is an excellent reflector of neutron especially curved surfaces for reflecting the neutrons back toward the implosion.

Obviously a bomb is far different then the fusion in a fusor but my question is when counting neutrons are you also taking into account those that are not radiated outward but reflected internally and if so how are these being accounted for. It would seem that a good number more "thank goodness" are contained by the SS enclosure.

Also is it beneficial to maintaining the fusion reaction, as it with a bomb to make use of contain and reflect the neutrons created? If so Taylor mentions the importance of a good neutron reflector. He recommends Glasstone's "Sourcebook on atomic Energy," under Nuclear Reactors: Reactor Moderators and Reflectors. He goes on the recommend the use of natural uranium, steel, copper, magnesium, lead, aluminum, beryllium, water, solder, or wax, as neutron reflectors. Such as two stainless steel mixing bowls lined with was and soldered together. A three-inch thickness of was will reflect as many neutrons as an inch and a half of steel. To get good reflection of neutrons you need two-inches of steel. However the best material for neutron reflection is beryllium which is the most poisonous nonradioactive inorganic material on earth. However it is one of the lightest elements and less dense then aluminum, but many more atoms per cubic centimeter making is a great reflector. Beryllium cost about a $100 a pound but is brittle and hard to work. If a SS sphere could be lined with beryllium it would reflect most all the neutrons back toward the plasma… then again the assumption which might be wrong on my part is that these neutrons are needed to maintain the fusion process?

One last point from the book at one point there was a need for more Tritium and the process of making it is very slow and comes mostly from reactors and cost about eight hundred thousand dollars per kilogram. So Taylor proposed putting a large blanket of lithium around a thermonuclear bomb and placing it under ten thousand feet of ice and setting it off. This would provide and underground lake of full of heavy isotopes. This idea didn’t fly, however he had support for ideas like this from folks like Freeman Dyson, John von Neumann and George Gamow.

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Brian McDermott
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Re: Neutron Reflection and Counting

Post by Brian McDermott » Tue Sep 27, 2005 11:06 am

Ted Taylor is an interesting man and "The Curve of Biding Energy" is an excellent book.

A "neutron reflector" does not reflect neutrons in the traditional sense like a mirror. It merely scatters them, and small fraction happen to be redirected in the direction from whence they came.

They are really inefficient, and the only reason they work in bombs is that the neutron flux is so high, it doesn't really matter. In a fusor, a reflector will be a regular moderator at best, or totally useless at worst. The stainless steel chamber does absolutely nothing in the way of shielding or reflecting neutrons.

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Richard Hull
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Re: Neutron Reflection and Counting

Post by Richard Hull » Tue Sep 27, 2005 3:47 pm

In fusion, the neutrons have no boosting or valuable effect whatsoever to the reaction itself. You really want them OUT of the chamber and this is indeed where they go. There is no need for a reflector at all in fusion energy systems. An absorber is far more important as this is where the energy is taken from the neutrons and converted to electricity via any number of methods. (most often steam.)

Reflector/scatterers are useful if you are making isotopes or are activating materials for research, but are just of no real value in energy production with normal fusion.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

3l
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Re: Neutron Reflection and Counting

Post by 3l » Tue Sep 27, 2005 3:52 pm

Hi Terrence:

A neutron is inelastically scattered into a partial circle by the reflector. It is critical for the explosive yield that neutrons stay in the bomb core as long as possible. more time more bang.
By Ted's use of reflectors the critical bomb size for a urainium bomb core went from cantalope size down to billard ball size.

Happy Fusoring!
Larry Leins
Fusor Tech

TLGNick
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Re: Neutron Reflection and Counting

Post by TLGNick » Tue Sep 27, 2005 11:00 pm

Brian, Richard, Larry,

Thanks for the feedback this was one of the details I was concerned with was the importance of the neutrons released from the reaction and if they were needed to help sustain the reaction, or if they were just a by product that was being measured to determine if fusion was occurring. Obviously sustaining the fusion reaction is the trick and later comes the conversion to useful energy worries. However, I seem to recall reading some place that steam from a fusion reactor to power turbines to produce electricity is not a very efficient method of using the energy produced; that at best it is perhaps 30% efficient. A more useful process would be to collect the energy directly to produce electricity, such as a grid in some fashion like the photo electric effect where the energy released is directly converted.

Has anyone attempted to convert the radiated energy to electricity? Granted the output would be less then the input as it stands with the current technology, but it might be of value to measure output vs. the input as a reference point just to see how far one is from breakeven. I suspect it would trail of rather fast, exponentially, once the kick-start voltage was removed, but there would seem to be some value to capturing and measuring the out from the reaction. I’m sure it is no trivial matter. Any thoughts on this?

And yes Brian, Ted Taylor is a really interesting man I'm glad to hear someone besides me read the book. :^)

Todd Massure
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Re: Neutron Reflection and Counting

Post by Todd Massure » Tue Sep 27, 2005 11:26 pm

direct energy conversion has been discussed many times on these forums and is a continuing area of interest for many of us. There was a thread going very recently in fact with some ideas thrown out I think the heading was "direct energy conversion".
The consensus here seems to be
1) Fusion break even is the goal first, then power conversion. I don't know if there is any way fission energy could be converted directly, or any energy from neutrons for that matter.
2) It's easier said than done

Neutrons are frustrating little buggers...Tiny in comparison to even the smallest atom and totally chargeless, in other words not affected by electric or magnetic fields. This makes them incredibly hard to harness or manipulate. They are essential to the chain reaction in fission as they are the blade that splits the atom. Directing their energy back into a fusion reaction area could help maintain a higher thermal energy, but from Richard's comments it sounds like it actually becomes a hinderance to the reaction, I'm not clear on the mechanism, but I imagine it would scatter the fuel, and / or make the density of fusable fuel less, by essentially thinning out or "muddying up" the fuel.

-Todd

TLGNick
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Re: Neutron Reflection and Counting

Post by TLGNick » Wed Sep 28, 2005 12:55 am

Thanks Todd I'll check out the treads. My apologies if I'm treading ground already covered, forums like this can me a little hard to catch-up on what has or has not been discussed so I appreciate the nudge. I was looking for a subject matter under vacuum technology the other day and was flooded with references such that I forgot just what it was I was looking for. :^)

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Richard Hull
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Re: Neutron Reflection and Counting

Post by Richard Hull » Wed Sep 28, 2005 2:07 pm

Neutrons neither help nor hinder fusion reactions. The reaction zone is so small and the fuel so thin that 99.999999999999999999% of all neutrons produced just sail through the reaction, the fuel, and the shell. NONE stay inside or do anything inside the fusor. Nor would they do anything cool or useful if they did.

In theory, if they were thermalized and continouosly locked inside the chamber they would have some slight chance of fusing with other reacted or reaction gases provided the cross sections and applied potentials are just correct, but that is the longest shot in the universe. It is best that they just leave which is what they are more than willing to do.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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